The present invention relates generally to network interfacing, and more particularly, to a method for determining a decimation pattern in a network communications receiver.
The transmission of various types of digital data between computers continues to grow in importance. The predominant method of transmitting such digital data includes coding the digital data into a low frequency baseband data signal and modulating the baseband data signal onto a high frequency carrier signal. The high frequency carrier signal is then transmitted across a network physical transmission medium such as electrical cable, fiber optic, RF, or other medium to a remote computing station.
At the remote computing station, the high frequency carrier signal must be received and demodulated to recover the original baseband signal. In the absence of any distortion across the network medium, the received signal would be identical in phase, amplitude, and frequency to the transmitted carrier and could be demodulated using known mixing techniques to recover the baseband signal. The baseband signal could then be recovered into digital data using known sampling algorithms.
One problem with such networks is that the physical medium and network topology tend to distort the high frequency carrier signal. Branch connections and different lengths of such branches cause reflections of the transmitted signal. Such problems are even more apparent in a network which uses home telephone wiring cables as the network physical medium. The typical wiring of the telephone network is designed for the xe2x80x9cplain old telephone servicexe2x80x9d (POTS) signals in the 3-10 kilohertz frequency and are not designed for transmission of high frequency carrier signals in afrequency range greater than 1 MHz. The high frequency carrier signal is also distorted by transients in wiring characteristics due to on-hook and off-hook switching and noise pulses of the POTS (e.g. ringing). The high frequency carrier is further distorted by spurious noise caused by electrical devices operating in close proximity to the xe2x80x9ccablexe2x80x9d medium.
Such distortion of frequency, amplitude, and phase of the high frequency carrier signal degrades network performance and tends to impede the design of higher data rate networks. Known techniques for compensating for such distortion and improving the data rate of a network include complex modulation schemes and frequency diversity.
Utilizing a complex modulation scheme such as quadrature amplitude modulation (QAM), both the amplitude and phase of the high frequency carrier are modulated to represent I and Q components of a baseband signal. Referring to FIG. 1, a 4-QAM modulation constellation 10 is shown. In operation, each data symbol is represented by an I-value of +1 or xe2x88x921 and a Q-value of +1 or xe2x88x921 such that the data symbol can be represented by one of the four states 12(a)-12(d) in constellation 10. Each constellation pointy 12(a)-12(d) represents a unique combination of carrier amplitude and phase. For example, constellation point 12(a) represents a carrier amplitude of 14 and a carrier phase 16.
FIG. 2 illustrates the utilization of frequency diversity by transmitting the same data in three mutually exclusive sub-spectra 18(a)-18(c) of the transmission band 20. Therefore, if a portion of the band is distorted (e.g. one or more of the sub-spectra 18(a)-18(c)), the data may still be recovered at the receiver from a less distorted portion of the sub-spectra 18(a)-18(c). For example, a data signal modulated onto a 7 MHz carrier utilizing 6 MHz of bandwidth may include three mutually exclusive sub-bands 18(a)-18(c) centered at 5 MHz, 7 MHz and 9 MHz.
One approach to demodulating such complex signals is to use filters implemented by digital signal processing (DSP), which provides for a convenient way of varying filter coefficients for each transmission to accommodate carrier distortion as detected in the particular time frame in which the data is being transmitted. Using such approach, the receiver compares the distorted received signal representing a known preamble transmission (prior to the data transmission) to the undistorted waveform of the preamble and determines the appropriate filter coefficients for recovery of the received signal. Such filter coefficients are then used for receiving the data transmission.
In accordance with DSP technology, the high frequency carrier is typically sampled with an A/D converter ata rate that is at least 4 times the carrier frequency. Assuming a carrier frequency on the order of 7 MHz, the sampling rate will be on the order of 30 MHz. A problem associated with processing digital samples at such rates to demodulate a complex modulated carrier, and to process mutually exclusive sub-bands of a frequency diverse system, is that very large and costly DSPs would be required. A known solution to reduce the hardware size and complexity is to use decimation filters to reduce the sampling rate. A problem associated with decimation filters is that they ignore, or decimate, a significant portion of the samples, which can significantly degrade the quality of the reduced frequency signal represented by the retained samples. Therefore, there is a need for a decimation filter and an associated method for selecting samples for retention that do not suffer the disadvantages associated known systems.
A first aspect of the present invention is to provide a method of selecting a decimation phase of a decimation filter to filter a sequence of samples, comprising the steps of: a) determining a plurality of phase groups; b) assigning each sample to one of the plurality of phase groups; c) calculating a phase strength value for each phase group; and d) selecting the phase group with the greatest phase strength value as the selected decimation phase. The quantity of the plurality of phase groups may be equal to a decimation factor of the decimation filter. The phase strength value of a phase group may represent the sum of sample magnitudes in the phase group of the sum of the squares of sample magnitudes in the phase group.
The sequence of samples may represent a frequency diverse redundant data signal comprising a plurality of adjacent sub spectrums and a decimation factor of the decimation filter may be equal to an input sample frequency divided by a frequency difference between the center of two adjacent sub spectrums. The phase strength value of a phase group may represent the sum of sample magnitudes in the phase group of the sum of the squares of sample magnitudes in the phase group. The data signal may be a baseband signal or other than baseband.